The invention relates to a method for manufacturing a large-scale-integration semiconductor device and more particularly to a process of depositing an Al--Cu thin film in small via holes at a high throughput.
With large-scale-integration semiconductor devices, for example, current 256 Mb DRAMs, it is required to fill in a very small via hole for the formation of the second level (the topmost layer) of multilevel inter-connections. An Al--Cu sputtering process, which allows the second level of interconnection and contact between the first and second levels of interconnections to be formed in one step, can reduce both the process time and cost and thus has been employed in the BEOL (Back End of Line) wiring process for 64-Mb DRAMs and CMOS logic circuits.
With 256-Mb DRAMs, however, the via hole size is scaled down to less than 0.8 .mu.m. It is therefore impossible for the conventional Al--Cu sputtering process to completely fill in the via hole without leaving a void. The void significantly reduces the metal line reliability. Early failures are observed on all hardware in reliability tests in which current and temperature are applied to the second level of metal and the contact between the first and second levels of metal. Since high-temperature (above 450.degree. C.) hot reflow process cannot be carried out on the topmost layer of metal in particular, the void is difficult to remove by means of post-treatment as well.
As techniques for filling in small and high-aspect-ratio via holes to form interconnect layers while suppressing the formation of the void, long throw sputtering and collimated sputtering are known. The long throw sputtering is a process which deposits a film of metal onto the bottom of a deep via hole by making the target-to-wafer distance long to decrease the horizontal components of metal particles that arrive at the wafer. The long throw sputtering process is described, for instance, in Journal of Vacuum Science and Technology B Vol. 13(4) (1995) p.1906 N. Motegi et al. In comparison with the collimated sputtering process, the long throw sputtering process has advantages that a relatively high sputtering rate and a good target efficiency can be achieved.
On the other hand, the collimated sputtering is a process which uses a collimator placed between the target and the wafer and thereby allows only vertical components of metal particles from the target to pass through the collimator and reach the bottom of the deep contact hole. This collimated sputtering process is described, for example, in U.S. Pat. No. 4,724,060, Journal of Vacuum Science and Technology A Vol. 9(2) (1991) p. 261 S. M. Rossnagel et al., and Thin Solid Films Vol. 247 (1994) p.104 B. Vollmer et al. The collimated sputtering process has good center-edge uniformity.
In the process of depositing the second level (the top layer) of metal for 256-Mb DRAMs, the long throw sputtering by which an Al--Cu thin film is deposited at a low temperature of about 150.degree. C. in a long time or the collimated sputtering is used. Both the long throw sputtering and collimated sputtering processes can fill in the via hole without leaving a void and pass the reliability test.
However, the long throw sputtering and collimated sputtering processes employ only vertical components of metal particles emitted from the target and thus require a long time to deposit a metal film as compared with the usual sputtering process. In order to deposit an Al--Cu film having a thickness of the order of 1 .mu.m, a deposition time of six to seven minutes is required. For this reason, the wafer throughput becomes low as compared with the case where the usual sputtering process is used, leading to an increase in manufacturing cost. In addition, with the collimated sputtering, metal is also deposited onto the collimator, so that the collimator often clogs. The collimator clogging varies the sputtering rate. In such a case, the collimator must be replaced with a new one and the downtime due to collimator replacement further reduces the throughput.